THOUSANDS of police marksmen will be on London’s streets and rooftops again today after warnings that another team of suicide bombers is plotting a third attack on the capital.

The new group is believed to be made up of British Muslims who were understood to be close to staging an attack on the Underground network last week. According to security sources the men are thought to be of Pakistani origin but born and brought up in this country. They have links with the Leeds-based terrorist cell that staged the July 7 attacks, in which 52 innocent people died.

Even with the transport system so heavily guarded, police and intelligence sources believe that the bombers are intent on once more attacking London’s bus and Underground network. Another multiple suicide strike is also intended to demonstrate how the network can call on more recruits. The men are said to have access to explosives.

US security sources said yesterday that this third group of would-be bombers met at Finsbury Park mosque in North London, where some of the July 7 terrorists are also known to have stayed. There are reports that this team originally planned to strike last Thursday, which is why more than 6,000 police, half of them armed, were present at Underground stations. Scotland Yard said at the time that this exercise, the biggest since the Second World War, was to test their resources and reassure a nervous public.

As commuters return to work today police chiefs say that the arrest of five suspected bombers in house raids in Birmingham, London and Rome has not ended this threat. Deputy Assistant Commissioner Peter Clarke, head of the anti-terrorist branch, said: “The threat remains and is very real.”

There is concern among ministers and police at how long officers can continue such an intensive operation to “lock down” London while a threat remains. Although reinforcements have been brought in and leave has been cancelled, resources are stretched to keep up the guard on the capital, which is costing £500,000 a day. Sir Ian Blair, the Metropolitan Police Commissioner, admitted that his officers were “very, very tired”.

While the priority is to thwart another strike, police are still investigating links between the attacks on July 7 and the botched operation a fortnight later. They are also hunting for what officers describe as “key logistical players” behind the attacks.

Seven more people — six men and a woman — were arrested in raids in Brighton yesterday, bringing the number of people under arrest in Britain to 18. A Scotland Yard spokesman said: “This is a further indication of the fact this is a fast-moving investigation and we continue to progress. We are searching for other people in connection with this ongoing inquiry.

“There were quite a few other people involved in the incidents of the 7th and the 21st. It’s extremely likely there will be other people involved in harbouring, financing and making the devices.”

The major link between the two sets of bombers is that the alleged leaders of both groups attended Finsbury Park mosque. Experts are studying similarities between the bombs used on July 7 and 21.

Anti-terrorism officers are still questioning four of the failed bombers at Paddington Green police station while a fifth member of the team is being interrogated in Rome.

Hussain Osman, who tried to blow up a Tube train at Shepherds Bush, told Italian police that the devices were only meant to scare passengers, not injure them. Scotland Yard dismissed that claim as “nonsense”.

The devices, hidden in rucksacks, were studded with razor sharp nails and only failed to explode because of a clumsy mistake by the bombmaker. Sir Ian Blair said that the bombs were designed to kill and that London had a lucky escape.

Ethiopian-born Hussain, 27, who has a British passport, claimed that the plot was orchestrated by another of those arrested on Friday, Muktar Said-Ibrahim. Hussain said that he had been recruited in an underground gym in Notting Hill.

Immigration officials are trying to find out how he managed to slip out of Waterloo station on a Eurostar train to Paris and make way to Italy where he met his brother, who lives in Rome. Officials want to know why Hussain, who says his real name is Hamdi Isaac and who has Italian citizenship, came to Britain posing as a Somali asylum-seeker in 1996.

There were reports last night that Muktar Said-Ibrahim, the suspected ringleader of the July 21 plot, was seen in Rome several weeks before the failed attacks. A mother and daughter living downstairs from the suburban flat where Hussain Osman was arrested on Friday, said that they had recognised Said-Ibrahim from footage of his arrest in London.

Two of Hussain’s brothers who live in Italy are also being held. One is accused of sheltering him; the second was picked up yesterday in the northern town of Brescia.

Italian police say they are using Hussain’s phone records to unpick the international network that has been helping him. Alfredo Mantovano, an Interior Ministry official, said that the network “confirms the presence in our country of autonomous Islamic cells . . . which could represent a concrete threat.” Italy is worried that it is the next target for Islamic terrorists.